Low concentration electrolyte with non-solvating cosolvent enabling high-voltage lithium metal batteries

Regulating Electrolyte Solvation Structures via Diluent-Solvent Interactions for Safe High-Voltage Lithium Metal Batteries

Local high concentration electrolytes (LHCEs) have been proved to be one of the most promising systems to stabilize both high voltage cathodes and Li metal anode for next-generation batteries. However, the solvation structures and interactions among different species in LHCEs are still convoluted, which bottlenecks the further breakthrough on electrolyte development. Here, it is demonstrated that the hydrogen bonding interaction between diluent and solvent is crucial for the construction of LHCEs and corresponding interphase chemistries. The 2,2,2-trifluoroethyl trifluoromethane sulfonate (TFSF) is selected as diluent with the solvent dimethoxy-ethane (DME) to prepare a non-flammable LHCE for high voltage LMBs. This is first find that the hydrogen bonding interaction between TFSF and DME solvent tailors the electrolyte solvation structures by weakening the coordination of DME molecules to Li+ cations and allows more participation of anions in the first solvation shell, leading to the formation of aggregates (AGGs) clusters which are conducive to generating inorganic solid/cathodic electrolyte interphases (SEI/CEIs). The proposed TFSF based LHCE enables the Li||NCM811 (LiNi0.8Mn0.1O2) batteries to realize >80% capacity retention with a high average Coulombic efficiency of 99.8% for 230 cycles under aggressive conditions (NCM811 cathode: 3.4 mAh cm-2, cut-off voltage: 4.4 V, and 20 µm Li foil).

An Ultralow-concentration and Moisture-resistant Electrolyte of Lithium Difluoro(oxalato)borate in Carbonate Solvents for Stable Cycling in Practical Lithium-ion Batteries

The electrolyte concentration not only impacts the battery performance but also affects the battery cost and manufacturing. Currently, most studies focus on high-concentration (>3 M) or localized high-concentration electrolytes (~1 M); however, the expensive lithium salt imposes a major concern. Most recently, ultralow concentration electrolytes (<0.3 M) have emerged as intriguing alternatives for battery applications, which feature low cost, low viscosity, and extreme-temperature operation. However, at such an early development stage, many works are urgently needed to further understand the electrolyte properties. Herein, we introduce an ultralow concentration electrolyte of 2 wt % (0.16 M) lithium difluoro(oxalato)borate (LiDFOB) in standard carbonate solvents. This electrolyte exhibits a record-low salt/solvent mass ratio reported to date, thus pointing to a superior low cost. Furthermore, this electrolyte is highly compatible with commercial Li-ion materials, forming stable and inorganic-rich interphases on the lithium cobalt oxide (LiCoO2) cathode and graphite anode. Consequently, the LiCoO2-graphite full cell demonstrates excellent cycling performance. Besides, this electrolyte is moisture-resistant and effectively suppresses the generation of hydrogen fluoride, which will markedly facilitate the battery assembly and recycling process under ambient conditions.

Reactivation of an air-passivated lithium metal anode through halogen regulation

A comprehensive study was conducted to investigate the failure mechanism of the lithium metal anode (LMA) in air. Simultaneously, an effective reactivation strategy was developed using halogen regulation. Specifically, iodine treatment converts the passivation layer of the exposed Li into LiI with fast Li+ transport ability, thereby improving the electrochemical performance.

Trimming the Degrees of Freedom via a K+ Flux Rectifier for Safe and Long-Life Potassium-Ion Batteries

High degrees of freedom (DOF) for K+ movement in the electrolytes is desirable, because the resulting high ionic conductivity helps improve potassium-ion batteries, yet requiring support from highly free and flammable organic solvent molecules, seriously affecting battery safety. Here, we develop a K+ flux rectifier to trim K ion's DOF to 1 and improve electrochemical properties. Although the ionic conductivity is compromised in the K+ flux rectifier, the overall electrochemical performance of PIBs was improved. An oxidation stability improvement from 4.0 to 5.9 V was realized, and the formation of dendrites and the dissolution of organic cathodes were inhibited. Consequently, the K||K cells continuously cycled over 3,700 h; K||Cu cells operated stably over 800 cycles with the Coulombic efficiency exceeding 99%; and K||graphite cells exhibited high-capacity retention over 74.7% after 1,500 cycles. Moreover, the 3,4,9,10-perylenetetracarboxylic diimide organic cathodes operated for more than 2,100 cycles and reached year-scale-cycling time. We fabricated a 2.18 Ah pouch cell with no significant capacity fading observed after 100 cycles.

Insights into the solvation chemistry in liquid electrolytes for lithium-based rechargeable batteries

Lithium-based rechargeable batteries have dominated the energy storage field and attracted considerable research interest due to their excellent electrochemical performance. As indispensable and ubiquitous components, electrolytes play a pivotal role in not only transporting lithium ions, but also expanding the electrochemical stable potential window, suppressing the side reactions, and manipulating the redox mechanism, all of which are closely associated with the behavior of solvation chemistry in electrolytes. Thus, comprehensively understanding the solvation chemistry in electrolytes is of significant importance. Here we critically reviewed the development of electrolytes in various lithium-based rechargeable batteries including lithium-metal batteries (LMBs), nonaqueous lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), lithium-oxygen batteries (LOBs), and aqueous lithium-ion batteries (ALIBs), and emphasized the effects of interactions between cations, anions, and solvents on solvation chemistry, and functions of solvation chemistry in different types of electrolytes (strong solvating electrolytes, moderate solvating electrolytes, and weak solvating electrolytes) on the electrochemical performance and redox mechanism in the abovementioned rechargeable batteries. Specifically, the significant effects of solvation chemistry on the stability of electrode-electrolyte interphases, suppression of lithium dendrites in LMBs, inhibition of the co-intercalation of solvents in LIBs, improvement of anodic stability at high cut-off voltages in LMBs, LIBs and ALIBs, regulation of redox pathways in LSBs and LOBs, and inhibition of hydrogen/oxygen evolution reactions in LOBs are thoroughly summarized. Finally, the review concludes with a prospective outlook, where practical issues of electrolytes, advanced in situ/operando techniques to illustrate the mechanism of solvation chemistry, and advanced theoretical calculation and simulation techniques such as "material knowledge informed machine learning" and "artificial intelligence (AI) + big data" driven strategies for high-performance electrolytes have been proposed.

Improving performances of the electrode/electrolyte interface via the regulation of solvation complexes: a review and prospect

The electrode/electrolyte interface (EEI) is a research hotspot in lithium-ion batteries, while the electrolyte solvation complex can be regarded as a factor that cannot be ignored in determining the performance of the EEI. From the perspective of the electrolyte solvation complex, this review summarizes the effects of solvation complexes on the composition of an EEI film and the Li⁺ desolvation process, and further clarifies the internal mechanism of the electrolyte composition controlling solvation chemistry. Finally, combined with doubtful points that are not comprehensively considered in the regulation of solvated complexes, this review puts forward some cutting-edge views, which are of great significance for future guidance in improving the performance of lithium-ion batteries.

Stable Solvent-Derived Inorganic-Rich Solid Electrolyte Interphase (SEI) for High-Voltage Lithium-Metal Batteries

The inorganic-rich solid electrolyte interphase (SEI) has attracted wide attention due to its good compatibility with the lithium (Li) metal anode. Herein, a stable solvent-derived inorganic-rich SEI is constructed from a hydrofluoroether-diluted low-concentration electrolyte, which simultaneously possesses the merits of nonflammability and low cost (0.5 M LiPF₆). The addition of hydrofluoroether enhances the coordination strength between Li⁺ and solvents, altering the decomposition path of solvents to yield more Li₂O. The abundant Li₂O crystals endow the SEI with improved passivating ability and ion conductivity. The 30 μm Li|NCM523 (3.8 mAh cm^-2) batteries with solvent-derived Li₂O-rich SEI deliver 96.1% capacity retention after 200 cycles. Notably, a 1.1 Ah Li|NCA pouch cell delivers an energy density of 374 Wh kg^-1 and achieves 45 stable cycles. This study points out that tuning the decomposition of solvents provides a new approach to construct stable inorganic-rich SEI for practical Li-metal batteries.

Electrolyte Engineering for High-Voltage Lithium Metal Batteries

High-voltage lithium metal batteries (HVLMBs) have been arguably regarded as the most prospective solution to ultrahigh-density energy storage devices beyond the reach of current technologies. Electrolyte, the only component inside the HVLMBs in contact with both aggressive cathode and Li anode, is expected to maintain stable electrode/electrolyte interfaces (EEIs) and facilitate reversible Li⁺ transference. Unfortunately, traditional electrolytes with narrow electrochemical windows fail to compromise the catalysis of high-voltage cathodes and infamous reactivity of the Li metal anode, which serves as a major contributor to detrimental electrochemical performance fading and thus impedes their practical applications. Developing stable electrolytes is vital for the further development of HVLMBs. However, optimization principles, design strategies, and future perspectives for the electrolytes of the HVLMBs have not been summarized in detail. This review first gives a systematical overview of recent progress in the improvement of traditional electrolytes and the design of novel electrolytes for the HVLMBs. Different strategies of conventional electrolyte modification, including high concentration electrolytes and CEI and SEI formation with additives, are covered. Novel electrolytes including fluorinated, ionic-liquid, sulfone, nitrile, and solid-state electrolytes are also outlined. In addition, theoretical studies and advanced characterization methods based on the electrolytes of the HVLMBs are probed to study the internal mechanism for ultrahigh stability at an extreme potential. It also foresees future research directions and perspectives for further development of electrolytes in the HVLMBs.